In recent years, electrophotographic apparatuses, which commonly were used in offices, have been used increasingly for personal purposes, and there is a growing demand for technologies that can achieve, e.g., a small size, a high speed, high image quality, or high reliability for those apparatuses.
During the formation of color images, toner may adhere to the surface of a fixing roller and cause offset. Therefore, a large amount of oil or the like should be applied to the fixing roller, which makes the handling or configuration of the equipment more complicated. Thus, oilless fixing (no oil is used for fixing) is required to provide compact, maintenance-free, and low-cost equipment. To achieve the oilless fixing, e.g., the configuration of toner in which a release agent (wax) with a sharp melting property is added to a binder resin is being put to practical use.
However, such toner is very prone to a transfer failure or disturbance of the toner images during transfer because of its strong cohesiveness. Therefore, it is difficult to ensure the compatibility between transfer and fixing. In the case of two-component development, spent (i.e., the adhesion of a low-melting component of the toner to the surface of a carrier) is likely to occur due to heat generated by mechanical collision or friction between the particles or between the particles and the developing unit. This decreases the charging ability of the carrier and interferes with a longer life of the developer.
Japanese Patent No. 2801507 (Patent Document 1) discloses a carrier for positively charged toner that is obtained by introducing a fluorine-substituted alkyl group into a silicone resin of the coating layer. JP 2002-23429 A (Patent Document 2) discloses a coating carrier that includes conductive carbon and a cross-linked fluorine modified silicone resin. This coating carrier is considered to have high development ability in a high-speed process and maintain the development ability for a long time. While taking advantage of the superior charging characteristics of the silicone resin, the conventional technique uses the fluorine-substituted alkyl group to obtain properties such as slidability, releasability and repellency, to increase resistance to wearing, peeling or cracking, and further to prevent spent. However, the resistance to wearing, peeling or cracking is not sufficient. Moreover, when the negatively charged toner is used, the amount of charge is too small, although the positively charged toner may have an appropriate amount of charge. Therefore, a significant amount of the reversely charged toner (positively charged toner) is generated, which leads to fog or toner scattering. Thus, the toner is not suitable for practical use.
With pulverization and classification of the conventional kneading and pulverizing processes of toner, the actual particle size can be reduced to only about 8 μm in view of the economic and performance conditions. At present, various methods are considered to produce toner having a smaller particle size. In addition, a method for achieving the oilless fixing also is considered, e.g., by adding a release agent (wax) to the resin with a low softening point during melting and kneading. However, there is a limit to the amount of wax that can be added, and increasing the amount of wax can cause problems such as low flowability of the toner, transfer voids, and fusion of the toner to a photoconductive member.
Therefore, various ways of polymerization different from the kneading and pulverizing processes have been studied as a method for producing toner. For example, toner may be produced by suspension polymerization. However, the particle size distribution of the toner is no better than that of the toner produced by the kneading and pulverizing processes, and in many cases further classification is necessary. Moreover, since the toner is almost spherical in shape, the cleaning property is extremely poor when the toner remains on the photoconductive member or the like, and thus the reliability of the image quality is reduced.
Also, toner may be produced by emulsion polymerization including the following steps: preparing an aggregated particle dispersion by forming aggregated particles in a dispersion of at least resin particles; forming adhesive particles by mixing a resin particle dispersion in which resin fine particles are dispersed with the aggregated particle dispersion so that the resin fine particles adhere to the aggregated particles; and heating and fusing the adhesive particles together.
JP 10 (1998)-198070 (Patent Document 3) discloses a process of preparing a liquid mixture by mixing at least a resin particle dispersion in which resin particles are dispersed in a surface-active agent having a polarity and a colorant particle dispersion in which colorant particles are dispersed in a surface-active agent having a polarity. The surface-active agents included in the liquid mixture have the same polarity, so that toner for electrostatic charge image development with high reliability and excellent charge and color development properties can be produced in a simple and easy manner.
JP 10 (1998)-301332 (Patent Document 4) discloses that the release agent includes at least one kind of ester composed of at least one selected from higher alcohol having a carbon number of 12 to 30 and higher fatty acid having a carbon number of 12 to 30, and the resin particles include at least two kinds of resin particles with different molecular weights. This can provide toner with an excellent fixing property, color development property, transparency, and color mixing property.
However, when the dispersibility of the release agent added is lowered, the toner images melted during fixing are prone to have a dull color. This also decreases the pigment dispersibility, and thus the color development property of the toner becomes insufficient. In the subsequent process, when resin fine particles further adhere to the surface of an aggregate, the adhesion of the resin fine particles is unstable due to low dispersibility of the release agent or the like. Moreover, the release agent that once was aggregated with the resin is liberated into an aqueous medium. Depending on the polarity or the thermal properties such as a melting point, the release agent may have a considerable effect on aggregation. Further, a specified wax is added in a large amount to achieve the oilless fixing.
When particles are formed by an aggregation reaction in the medium that contains at least a certain amount of wax, the particle size increases with heat treatment time. Therefore, it is difficult to produce small particles having a narrow particle size distribution.
The use of a release agent may achieve the oilless fixing, reduce fog during development, and improve the transfer efficiency. However, such a release agent prevents uniform mixing and aggregation of the resin particles with pigment particles in the aqueous medium during manufacture. Thus, the release agent tends to be not aggregated but suspended in the medium, and aggregated and fused particles are likely to be coarser due to the effect of the release agent.
Patent Document 1: Japanese Patent No. 2801507
Patent Document 2: JP 2002-23429 A
Patent Document 3: JP 10(1998)-198070 A
Patent Document 4: JP 10(1998)-301332 A